The present invention is concerned with organic materials, in particular with pleochroic dyes in solution with liquid crystal materials e.g. for electro-optic display applications.
Liquid crystal materials are well known organic materials which display phases, known as liquid crystal phases or mesophases, having a degree of molecular ordering intermediate between that of the fully ordered crystalline solid state and the fully disordered isotropic liquid state.
Electro-optical devices incorporating liquid crystal materials are well known and widely used as digital displays in such applications as watches, calculators and digital voltmeters. These devices utilise the optical contrast when an electric field is applied across a thin insulating film of suitable liquid crystal material. The molecules of the material (in a liquid crystal phase at the temperature of operation) are re-orientated by the field causing a change in an optical property of the part of the film where the field is applied, e.g. a change in ambient light scattering or transmissivity.
Liquid crystal materials have the property that their molecules can impose their ordering upon the molecules of other suitable dopant materials incorporated within them. This property is the basis of so-called "guest-host" devices e.g. display devices in which the host liquid crystal material and its guest material have one molecular configuration in the absence of an applied electric field and another molecular configuration when an electric field is applied across the material. The guest material is usually a pleochroic dye, which is a dye whose molecular absorption properties vary with the orientation of the electric vector of light incident upon its molecules.
The presence of such a dye can be used to enhance the contrast between the off state (with no electric field applied) and the on state (with electric field applied) of a liquid crystal display because the orientation of the dye molecules is in effect switchable by the effect of the applied electric field on the liquid crystal molecules and by the consequent re-orientation of the dye molecules by the guest-host effect.
As discussed further below there are several kinds of liquid crystal effects which can make use of the guest-host effect in electro-optical displays. These vary according to the kind of liquid crystal material used and the configuration of its molecules in the off state (e.g. as determined by the surface treatments of the substrates employed to contain the film of liquid crystal material).
In order to provide maximum contrast between the on and off states of a guest-host liquid crystal display it is important that the guest molecules adopt as closely as possible the time averaged orientation of the host molecules. However this is achieved only to a limited degree because of random thermal fluctuations. The degree to which the orientation varies from the ideal is measured by a quantity known as the order parameter S which is given by the following equation: EQU S=1/2(3 cos.sup.2 .theta.-1) Equation (1)
where cos.sup.2 .theta. is a time averaged term and .theta. is the instantaneous angular orientation of the molecules with respect to the time averaged orientation of the host molecules. The determination of the value of the order parameter S is well understood in the art; see for example the paper "A new absorptive mode reflective liquid crystal display device" by D. L. White and G. N. Taylor in the Journal of Applied Physics, 1974, 45 pages 4718 to 4723.
For perfect orientation the order parameter S is unity (that is .theta. is zero). Thus, pleochroic dyes for use in guest-host devices should have an order parameter in the liquid crystal host as high as possible (i.e. less than one but as near to one as possible). However they must also have adequate chemical, photochemical and electrochemical stability, e.g. stability when exposed to atmospheric contaminants, electric fields (as in device operation) and to ultra-violet radiation. They should not be ionic or have any ionisable character (otherwise the liquid crystal material will lose its insulating nature and conduct making the device useless). They must also have sufficient solubility in the host materials; although the concentration of guest pleochroic dye required for the desired effect is generally quite small (e.g. not more than a few percent of dye) nevertheless many pleochroic dyes are unsuitable because they are essentially insoluble in liquid crystal materials.
In U.K. Patent Application G.B. No. 2043097A, liquid crystal compositions have been proposed which can contain symmetrical dyes of the formula: ##STR2## wherein R.sub.A is hydrogen or a non-ionic substituent. These dyes are proposed for use in liquid crystal materials of positive dielectric anisotropy only in that U.K. Patent Application.
In U.K. Patent Application No. 2082196A there are proposed for use in liquid crystal materials symmetrical dyes of the formula: ##STR3## wherein R represents an optionally substituted alkyl or aryl radical, Q represents halogen, hydroxy, amino, alkylamino, dialkylamino, arylamino, nitro, alkyl or aryl and y represents an integer from 0 to 4.
The symmetrical dyes of formulae A and B, for example 1,5-bis(phenylthio)anthraquinone and 1,4,5,8-tetrakis(phenylthio) anthraquinone have high order parameters and adequate stability but in general their solubility in a number of liquid crystal materials is rather low for practical purposes; in particular they generally give somewhat poor contrast when used in an electro-optical display.